net: ethernet: mtk_eth_soc: use DEFINE_SHOW_ATTRIBUTE to simplify code

Use DEFINE_SHOW_ATTRIBUTE helper macro to simplify the code.
No functional change.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Link: https://lore.kernel.org/r/20220927111925.2424100-1-liushixin2@huawei.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>